Conventional integrated circuit production utilizes photolithography techniques to define a print on a wafer. Successive film photolithography and film growth or deposition creates a completed integrated circuit with many identical copies on the same wafer. Each copy is known as a die.
As integrated circuits have become smaller in size, the photolithographic process requires more sophistication in the alignment techniques and the resolution. Presently, photolithography processes utilize an instrument which steps and repeats in a manner such that the wafer is properly aligned, then a portion of it is exposed to the desired image from the appropriate photomask, then the wafer is stepped and aligned, after which the wafer is again exposed to the appropriate photomask.
This step and repeat alignment process utilizes a master image called a reticle. The image of the reticle is focussed through a lens onto a portion of the wafer.
Initially, individual reticles had only one die image on them. Therefore, it was necessary to step, align, and expose for each die on a wafer. Later, multiple dies were included on a single reticle, thereby allowing the exposure of many dies at a time. This reduced the number of steps, alignments, and exposures required for each wafer. Consequently, this procedure increased the capacity of each such instrument.
Such a system and process requires a separate reticle for each photolithographic step (i.e. for each mask level). Each time an instrument is changed to a different mask level the reticle must be changed. In other words, a new reticle must be procured for each photolithographic step. Reticles must be made under exacting conditions and are relatively expensive for use in making phototype wafers and other low volume production quantities (i.e. 25 to 5000 wafers).